Electro-mechanical relays have, of course, been used for many years. These relays have offered a substantial number of alternative operative conditions, such as normally on, normally off contacts, or a combination of these, trip means which may include a flag and may have a reset operation which could be electrical, mechanical, or both.
Normally, these relays have been individually designed and manufactured for the particular purpose and this often necessitates a number of different types of components and, particularly, from the point-of-view of the manufacturer, carrying substantial stocks-of relays having various characteristics.